The invention relates to a method of selecting routes in a communications network, as well as to a communications network in which such a method is applied. More specifically, the invention relates to a method of selecting routes in a communications network which comprises a number of mutually isolated areas at a first level, which areas are linked at a second level, in which network routing devices are provided at both levels for selecting routes, in which method for selecting a route from a first area to a second area use is made of a cost function and routing information stored in the routing devices, the routing information, stored in the routing devices of a particular area, relating to said area and to direct links from said area to routing devices of the second level, and at least some areas being directly linked to more than one routing device of the second level.
Several methods of selecting routes in communications networks are known. A method of the type described above is known as the "IS-IS protocol" (cf. reference [1]). For the transmission of routing information, this known routing protocol occupies the capacity of the network only to a very small extent since only a minimum amount of information relating to that part of the network situated outside the area is to be present within a particular area. The routing information present in a routing device of the first level, that is to say in a routing device located in a certain area, is essentially limited to the area concerned. The only routing information relating to the second level which is present within the areas relates to the direct links to routing devices of the second level (level two), that is to say the links to "adjacent" routing devices of the second level. In the IS-IS protocol, the routing information stored in the routing devices of the first level can therefore be said to relate exclusively to the respective area and the direct links of that area with routing devices of the second level.
As a result, although links within an area can be set up efficiently with the aid of the said protocol, there is the disadvantage that less efficient links are often selected mutually between the areas. In other words, because virtually no information relating to the second level is present at the first level and the costs of a total route can therefore not be foreseen, a less suitable link with the second level is often selected. Thus, an area cannot make any distinction between two routing devices, linked to said area, of the second level, whereas one of said routing devices at the second level may form a much shorter and/or cheaper link to another area. As a result, the routing at the second level is often not efficient, with the result that transmission capacity ("bandwidth") of the network is wasted.
It should be pointed out that the said cost function may be any function with the aid of which alternative routes may be compared. Thus, the cost function may be a function of the number of paths, the number of routing devices traversed, the number of kilometres, the delay encountered, the error probability, the throughput speed or the (usage) costs charged for a link. A combination of two or more such factors may also be used.
The publication "A Comparison between Two Routing Protocols: OSPF and IS-IS" (reference [2]) discloses the transfer of level 2 information to level 1 routing devices in the OSPF protocol. Due to this transfer of information, the OSPF protocol performs better with regard to interarea routes, as the routing devices of level 1 are able to choose better routes, i.e. routes having lower costs. In fact, in the OSPF protocol routing devices of a certain area contain information with respect to all possible destinations within the communications network, thus also with respect to destinations (partial networks with end users) in other areas. This does not only require a substantive amount of information to be stored in each routing device, but also reduces the effective capacity of the network, as updating this information requires large amounts of data to be transmitted between routing devices. The OSPF protocol, which provides almost complete routing information to all routing devices, has the benefit of an improved routing efficiency at the cost of a substantial overhead.